1. Field of the Invention
The present invention generally relates to an injection molding of a synthetic resin material and, more particularly, to an injection-molded product having a partly thin portion and method and apparatus for injection molding such a product.
2. Description of Prior Art
As is well known, one of the most suitable methods for mass production of synthetic resin made products is the injection molding which is carried out by injecting a molten resin into a cavity defined within a molding die. The injection molding permits a product having a relatively complicated shape to be manufactured easily because of less restrictions in a design of the cavity. In the conventional injection molding, however, various problems have been experienced in the manufacture of a product having a partly thin portion.
For example, a vanity case need be provided with a concave portion for accommodating cosmetics by reducing a wall thickness of the central portion of a receptacle. In injection molding such a product, the cavity has hitherto been defined as having two areas, one for the thick peripheral portion and the other for the thin central portion of the product, before the molten resin is injected into the cavity. This has been considered satisfactory if the ratio between the thickness of central portion and the thickness of peripheral portion is relatively small, that is, if the thickness of the former is not so small compared with the thickness of the latter. However, as the above ratio increases, that is, as the area of the cavity for the central portion becomes narrower, resistance to the flow of resin material becomes larger. Thus, the resin material injected from a side gate, which opens into the cavity at a periphery thereof, tends to fill up the peripheral area of the cavity before flowing into the narrow central area. This results in an occurence of a weld mark or line on the product at the opposite side of the gate, which spoils an external appearance and reduces a mechanical strength of the product. Furthermore, an extremely narrow central area of the cavity prevents the resin from filling it up because the flow of resin concentrates a pocket of air in the cavity to the center and increases the air pressure considerably. An attempt to increase an injection pressure for overcoming the air resistance has involved a burned on the product due to a burning of the compressed air.
The above problems might be solved by providing a pin point gate opening into the cavity at a center thereof, instead of the side gate. In this case, however, a gate mark is necessarily formed on a center of the product and therefore an applicability of the pin point gate is limited. Another and more important disadvantage is that the peripheral area of the cavity tends to be insufficiently charged with the resin so that a sink mark often occurs on the product. This is because the resin is first injected into the narrow area and then flows into the peripheral area with a considerable loss of an injection pressure.
Therefore, the conventional injection molding does not permit the ratio between the thickness of the thin portion and of the thick portion to be considerably increased. For example, in a vanity case of a usual size having a wall thickness of about 5 mm at the peripheral portion, the bottom wall defining the concave portion has been formed to have a thickness of about more than 2 mm, i.e. more than 40% of the peripheral portion. Thus, the entire case has been bulky and not satisfactory.
Accordingly, an object of the present invention is to provide a method and an apparatus for injection molding a product having a partly thin portion, which enable to reduce a thickness of the thin portion to a large extent while maintaining a sufficient thickness of the remaining portion.
Another object of the invention is to provide an injection molding method and apparatus which permits an efficient manufacture of a product having a partly thin portion with a good external appearance and strength.
A further object of the invention is to provide an injection-molded product in which a thickness of a thin portion can be as small as desired.